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Discussion Starter #1
Dear HK Forum users.

I've owned my HKMR762 for many years. Everything has been kept stock (except PWS FSC91 compensator) and shoot a variety of ammo (always brass) through it, including military surplus. I am meticulous about cleaning it after every use and throughout the years I may have shot a total of 1000 rounds through it.

A few years ago I had a really bad FTE which cut my range day short. The brass rim was sheared off by my ejector, leaving the entire casing still in the bore while trying to feed another round. This was a major headache and I kept a cleaning rod with me in case this happened again.

Fast forward to a few days ago, I had the same issue. Luckily, I was able to clear it up at the range (safely). I have a tendency to keep all of my brass to inspect further. This time, the ejector did not completely shear off the rim of the brass casing, but did bend it back quite a bit. Upon further inspection, I had a total of 12 rounds that had some kind marking from my ejector on it. It varied from a simple indention/ deep scratch from my ejector to real damage to the casing.

What is strange, is that I used two different sets of ammo, and only one appeared to have this issue (Hirtenberger .308 Winchester 146 Gr. FMJ (Bi-Metal) Brass Case Berdan Primed). Of all of the forums I've looked into, everyone touts on the quality and thickness of the Hirtenberger brass casing, yet, that was the only brass I had an issue with.

I thoroughly inspected my ar10, and even took it a step further and did a completely disassembly of the BCG and the bolt (I know HK says it's not user serviceable....it is) Everything is in tip-top shape. I know any sensible person would say it's obviously the type of ammo you are using.... But I purchased my HK with the thought that it would be able to handle any/ most factory loads & pressure. My best guess is that the hirtenbergers are substantially hotter causing a possible premature/ harder pull on the ejection port... But that is simply a guess. I'm seeking out the forum to see what your thoughts and experience. Unfortunately, I don't have pictures of the brass. But I cannot be the only one experiencing this issue...

Thank you!
 

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It would have helped tremendously if you had posted picture of your disassembled weapon, preferably uncleaned and after cleaning, rather than trophy shots that say nothing to an issue that seems to be an ammunition issue.

Also Hirtenberger is just typical 146gr ball, like the US M80 or German DM111, its not hot or of any significant quality. Its typical low energy light bullet ~2700-3000J (60cm test barrel) Surplus. If you think that is hot for a factory round, you should shoot 1000yd .308 factory match rounds like the RWS Target Elite that gets about 4100J energy from a 60cm test barrel. I have recently shot about 100 rounds of that pretty much maxed .308 load, and had no issues, so I would be fairly confident in saying that the MR308 works excellently on loads that are absolutely at the limit of .308 hotness.
 

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I would presume the ammo is bad then, possibly slightly corroded? The HK has a rather snug chamber for accuracy, maybe that is the reason.
 

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Discussion Starter #4
I would presume the ammo is bad then, possibly slightly corroded? The HK has a rather snug chamber for accuracy, maybe that is the reason.
I'm going to go to the range to shoot the ammo again this week and provide pictures of the rounds, BCG (before and after). Thank you for your help
 

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Hey, Ack.
You must be a patient man, for tolerating this problem for so long. Since no experts have offered a solution, I'll give you my "less-than-expert" thoughts. Extraction is affected by timing and gas pressure. If the ammo you're running is higher pressure, then perhaps the action is cycling a little earlier, before the gas pressure has reduced sufficiently for proper extraction. I don't think your gas port is adjustable, so the first thing I would do is check everything in the gas system. Have you called HK?
 

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Discussion Starter #6
Hi Rusty,

I think we are thinking the same thought (although your response is worded much better). to answer your question, no there is not an adjustable gas port. Also, not sure if you were aware that this is piston driven. I have not contacted HK, since I am well outside my warranty and not sure how much help they would be. But definitely worth a shot.

On another note, did you notice all of those yellow dots all over my BCG/ lugs? I have never seen that before until I took those macro shots, but it seems pretty dirty for a piston driven system firing about 200 total rounds.. maybe it's just camera that is magnifying the dirt and grime. Anyways. I will keep this thread posted as more info comes in, but definitely feel it has something to do with the timing as you stated...
 

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i'm going to give you my best educated opinion however this is only my opinion and i can't base it off any facts to back it up.

researching this round it appears it is not loaded hot but rather at the low end of the scale. it looks to me like the projectile is not clearing the barrel fast enough to allow the pressure to reduce before trying to extract the spent case. this extra pressure on the case combined with the weak or thin brass causes the rim to bend or crack and the case to get stuck. you can test this out by using a chrono to see what velocity the projectile is leaving the barrel at.

as far as the little balls in the chamber go, that looks like unspent powder to me (just my opinion again) and possible the primers are not igniting all the powder. this could be due to the age of the ammo or possible the ammo was subjected to humidity thereby weakening the primers.

all of this is only my educated guess and i have only been shooting about 30 years now so there are several people on here sharper than i am.

either way i would NOT be shooting any more of this batch of ammo if i were you.
 

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I had this happen with old West German milsurp ammo. This was some ammo that I bought in the 1980s and fired quite a bit through a 7.62 NATO Galil that I owned back then and an FN FAL without any issues.

The gun was new at the time. I had fired about a hundred rounds of quality US brass cased ammo through the gun, as well as some foreign mil surplus brass cased ammo. The MR762 and ended the day with a malfunction where the gun ripped the rim off a case of old West German 69-59 NATO 7.62x51ammo and would not be dislodge from the chamber. It happened with the first round of this ammo.

Here is what the case with the torn rim looked like:



Comparison of bottom of the cases, the Federal on left the West German on the right:




A side by side with a Federal case on the left and the failed to extract German case on the right:

 

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High speed footage has shown that in virtually any semi-auto AR style platform, the bullet is well out of the barrel but approximately 25 feet or so before the BCG even begins rearward movement.

The little yellow grains are burned and unburnt powder. I used to sweep up stuff that looks exactly like that at the end of every day when I used to work at a gunshop/indoor shooting range. I tested it, it burned. Something tells me that this ammunition doesn't exactly use high quality powders. Nor very consistent powder.

Take a pic of the primer of an unfired round and post it (I don't have any of that brand of ammunition on hand to see for myself). I would like to see how the primer looks before and after firing. The pics you posted show what I would call a slightly flattened primer (not the dimple from the firing pin) but not VERY flattened. The reason this is important is because if it were a case of the ammunition being too hot, one of the prime indicators is that the primers get forced rearward and flatten against the bolt face by the pressure in the chamber. In extreme cases, it will also blow the primer completely out of the rear of the case and let it rattle around inside your FCG, possibly causing failure to fire by not letting the hammer release.

I also stated that you clean your gun regularly. Do you use an actual dedicated chamber brush or just something like a boresnake or simply a bore brush on a rod?

Lastly, if you remember where the brass is landing when it does extract would be a clue. Over pressure causes excessive bolt speed and brass ejects far forward. Admittedly the dynamics of the HK system do not allow the typical AR15 ejection chart to be cross referenced perfectly though mostly due to the much stronger ejector spring in the HK guns. Not only direction of ejection, but strength of ejection. Does it have a healthy brass throw or does it kind of just dump out the side? Faster bolt speed means a much stronger throw, slower bolt speed makes the brass almost fall out of the ejection port.
 

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Discussion Starter #10
Thanks for sharing Bullpup. I'm glad i'm not the only one. I know a lot of people would simply state: "just stop using that particular ammo, nuff said", but I want to go after the why. Looking at the side by side comparison of the 308 vs the 7.62 I can see the neck slightly shorter on the 7.62. Not sure if this had anything to do with it. First time it happened to me I was very upset and frustrated... This now has morphed into addiction to understand the reasoning behind this. I know the MR762 is a very powerful gun, but sheering off that case rim so clean just speaks volume about the forces at play.

Have you had any other issues with the side sheered off?
 

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Discussion Starter #11 (Edited)
Hi InshallahTech,

I just did a very VERY unscientific test a few minutes ago.. Took all of the charge out of the Hirtenberger and put it in my fireplace. Did the same with a PPU charge. I burned them both. When lit, the Hirtenberger flame went up to about 22 inches. The PPU about 14 inches in height. The PPU burned almost 2x longer than the hirtenburger. Not sure if that tells you anything at all....


I did a lot of research online.. Didn't appear there was any high pressures exerted on any of the primers.. Everything looked good to me, but you be the judge...



I use a chamber brush (where it has a regular brass bristles for the actual barrel and has the extra long bristles near the bottom end to clean the actual lugs. I use Q-Tips w/ alcohol to clean the chamber and the lugs thoroughly, then finish with a nice coat of Ballistol.

I can't provide any good info regarding where the brass lands as I am in close quarters at the indoor ranges and typically the brass simply flies, hits the wall and then splatters somewhere.
 
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